The renal preglomerular microvasculature is critically involved in achieving the homeostatic functions of the kidneys, including the regulation of Na+ and H2O excretion. The primary Na+ retaining hormone, angiotensin II (AngII) elicits preglomerular (and postglomerular) vasoconstriction via engagement of AT1 receptors (AT1R), which provoke G protein-mediated phospholipase C (PLC) activation and the rise in intracellular Ca2+ concentration ([Ca2+]i) that fuels activation of the contractile apparatus. AngII also exerts mitogenic effects on vascular smooth muscle through AT1R-dependent transactivation of the epidermal growth factor receptor (EGFR). The EGFR is a receptor tyrosine kinase that can form a signaling complex with multiple downstream branches, including the Ras/MAP kinase pathway for increasing expression of pro-mitogenic gene products. Recently, EGFR tyrosine kinase activity has been implicated in eliciting contraction of vascular smooth muscle from large arteries. Our preliminary data indicate that renal afferent arteriolar contractile responses to AngII involve tyrosine kinase(s), including the EGFR tyrosine kinase, and that this process contributes to the [Ca2+]i response. The proposed work will address the hypothesis that AngII-induced contraction of renal preglomerular microvascular smooth muscle cells (PVSMCs) involves AT1R-mediated initiation of a complex signaling network that includes H2O2-mediated activation of Src family kinase(s) and subsequent EGFR transactivation. We will also investigate the roles of PLC(1 and PLC(1 in the AngII-induced signaling events that generate the characteristic bi-phasic [Ca2+]i response to the peptide. We further postulate that up-regulation of EGFR-dependent pathways in pro-mitogenic states (such as AngII-induced hypertension) will promote exaggerated contractile responsiveness to AngII. The validity of these postulates will be examined by addressing the following specific aims: 1) Establish the role of Src family kinases in AngII-induced signaling and contractile responses of PVSMCs; 2) Determine the role of H2O2 in AngII-induced contractile signaling; 3) Determine if the classical AT1R / PLC(1 / IP3-dependent Ca2+ mobilization event is a prerequisite for EGFR transactivation; 4) Determine if PLC(1 plays a role in the PVSMC contractile response to AngII, and if this occurs as a consequence of EGFR transactivation; and 5) Evaluate the postulate that chronic activation of AT1R-EGFR signaling pathways promotes accentuated preglomerular vasoconstrictor responsiveness to AngII. The experimental strategy will utilize molecular, pharmacological, biochemical and physiological approaches to clarify the role of tyrosine kinases in evoking agonist-induced constriction of the preglomerular microvasculature. By exposing the activity of specific tyrosine kinases in PVSMCs and their contributions to the contractile response to AngII, the successful completion of this project should advance our understanding of the regulation of preglomerular microvascular tone, an important determinant of peripheral resistance, Na+ excretion and arterial pressure under physiological and pathophysiological conditions. [unreadable] [unreadable] Lay Summary: Angiotensin II is a potent regulator of blood pressure, acting in part through effects on microscopic blood vessels in the kidney organ. This work is exploring the mechanisms through which angiotensin II rapidly contracts muscle cells in kidney microvessels, focusing on processes previously thought to occur only in slower-developing growth responses. Completion of this project should advance our understanding of angiotensin II-dependent regulation of kidney function and blood pressure in health and disease. [unreadable] [unreadable]